1. Field of Invention
The present invention is concerned with a microwave switch matrix, and more particularly, with a planar switch matrix, one in which all components are on one side of a substrate, and in which an air bridge is used to separate input lines from output lines.
2. Description of Prior Art
U.S. Pat. No. 4,316,159 issued Feb. 16, 1982 to P. T. Ho and assigned to applicant's assignee (hereinafter Ho's patent) discloses a planar microwave switch matrix that is one in which all signal conductors and all components are on one side of a substrate. Although not specifically mentioned in the patent, Ho's original disclosure from which the patent resulted, discloses bond wires where the input and output lines of the matrix cross. The use of bond wires at crossover points is a conventional construction method. Bond wires have been found suitable for frequencies below about 1 gigahertz but less suitable at the higher frequencies because the effect of different length wires is more significant at the higher frequencies and the bond wires are hand assembled Bond wires tend to be of different lengths when the assembly is done by different workers These different length bond wires produce non-uniformity in the operating characteristis from assembly to assembly. Further, the various power dividers and combiners and the switching components are all of passive design such that there is signal power loss in passing through the matrix.
An article entitled "Coupler Crossbar Microwave Switch Matrix" by P. T. Ho et al. (hereinafter Ho's article) published in the 1982 IEEE MTT-S Digest at pages 239-241 discloses a different microwave switch matrix not disclosed in the patent in which input lines are on one side of a substrate while output lines are on the other side of the substrate. Fifty-ohm (50-.OMEGA.) feedthrough technology is utilized to connect the lines on two sides of the substrate. That is lines on opposite surfaces of the substrate are connected by a conductor through an orifice in the substrate. It is more difficult to fabricate components and conductors on two sides of a substrate than on one side in a GaAs monolithic microwave integrated circuit form which is the preferred form of realization in the instant invention. GaAs is a fragile material and the substrate thickness is normally 100 micrometers (.mu.m). Moreover, 50-.OMEGA. feedthroughs on GaAs substrates are difficult to fabricate. When the circuit is on both sides of a fragile substrate, the packaging of such circuit is difficult. This problem is much less severe in circuits using Al.sub.2 O.sub.3 (Alumina) substrate (which P. T. Ho has used). Advantages of fabricating circuits on GaAs substrate in monolithic form relative to hybrid circuits fabricated on Al.sub.2 O.sub.3 substrate are well known. As in the Ho's patent the power dividers and combiners (hybrid couplers) in Ho's article are of passive design although the switches utilize field effect transistors such that overall gain can be achieved by the switch matrix.
H. J. Finlay et al. in an article entitled "The Prediction and Characterization of Coupling between Transmission Line Structures to Enhance CAD of MMIC GaAs Circuits" published in the Conference Proceedings of the 13th European Microwave Conference, Sept. 5-8, 1983 in Nuremberg, W. Germany at pages 363-368 discloses various air bridges without specifying any particular use for them. He concludes on page 364 in the second sentence preceding the section entitled "Comparison of Theories with Measurements" that "the degree of coupling is higher than predicted (low or no coupling is desired). . . due to the proximity of the orthogonal input and output lines". In particular, on page 368 an air bridge is illustrated in FIGS. 10(a), 10(b) and 11(b) which results in about -35 dB coupling (35 dB isolation) at 4 gigahertz. As illustrated in the frequency plot of FIG. 11(a). More isolation (on the order of 50 dB) is required for proper operation of a microwave switch matrix used in spot beam communication satellites which is a contemplated use for the instant invention.